In the past it has been proposed to provide the cover of a pressurizable vessel for a nuclear reactor of the type described above, which is adapted to surmount a cylindrical prestressed structure, with annular prestressing by cables extending circumferentially around the outer periphery or rim of the disk-shaped cover member. Annular prestressing in this sense denotes the application of a radially inward force upon the body of the cover as generated by a tension cable or cable assembly bearing inwardly on an annular surface of the structure.
It has been found that such prior-art cover-prestressing systems are disadvantageous in some respects, since the application of the prestress to the outer periphery of a large-diameter disk-shaped body does not provide a uniform prestressing force at interior points of the disk. It has been found to be impossible with such system, for example, to establish a predetermined measurable inward prestress at any particular region of the cover inwardly of the periphery at which the cables acted. The application of numerically uniform or predetermined prestress forces at substantially all points or regions of the cover has been found to be particularly necessary for prestressed pressurized vessels of the pot-boiler type.
I have also found, with continuing work in systems of the type described in the copending application Ser. No. 638,075, that most effective force transmission in the radial and peripheral directions for ease of assembly cannot be obtained solely with circular-segmental sections of the cover. Moreover, the versatility of the cover is greatly limited if its component parts are confined to arcuate or circular segments.